1. Technical Field
The present invention relates generally to communication signal processing and more particularly to systems and methods for adaptative multi-carrier code division multiple access modulation.
2. Description of Related Art
Code Division Multiple Access (CDMA) encodes data with a special code. CDMA provides greater capacity and security in a variety of communications systems such as radio systems, networking systems, and wireless communications systems. One limitation with CDMA is the difficulty in achieving high performance due to channel impairments. Orthogonal Frequency Division Multiplexing (OFDM) splits a data stream into a number of lower rate streams that are each transmitted simultaneously over carrier frequencies that are orthogonal to one another.
OFDM has been used in residential power line communications and in Asymmetric Digital Subscriber Line communications. One example of a power line communication system using OFDM uses a large number of carriers that is greater than 1000 to achieve 200 Mbps. Because of the large number of carriers, the OFDM communication system has a high peak-to-average ratio. This OFDM communication system is robust against delay spread (cyclic prefix) and has high throughput. One limitation with OFDM is the high cost to achieve this high throughput. This example of a residential power line communication system uses a 2-30 MHz band that has a high delay spread and is regulated for injection of high power spectral density (PSD). With a high PSD, dynamic notches or power suppression are typically implemented to avoid interferences over amateur radio communications. In this example, there is high spectral efficiency but increasing performance such as beyond 9 bit/Hz increases costs.
Combining the OFDM with the CDMA results in Multi-Carrier Code Division Multiple Access (MC-CDMA). In MC-CDMA, each symbol is spread over multiple carriers with a special code, or each symbol is spread over the same carrier over time. One limitation is that high performance cannot be achieved due to channel impairments.